Lithium-ion battery technology has enjoyed a lot of attention in recent years and provides the preferred portable battery for most electronic devices in use today. Such batteries are “secondary” or rechargeable which means they are capable of undergoing multiple charge/discharge cycles. Typically lithium-ion batteries are prepared using one or more lithium electrochemical cells containing electrochemically active materials. Such cells comprise an anode (negative electrode), a cathode (positive electrode) and an electrolyte material. When a lithium-ion battery is charging, Li+ ions de-intercalate from the cathode and insert into the anode. Meanwhile charge balancing electrons pass from the cathode through the external circuit containing the charger and into the anode of the battery. During discharge the same process occurs but in the opposite direction.
Various electrochemically active materials have been suggested for use as the cathode materials, for example LiCoO2, LiMn2O4 and LiNiO2, see U.S. Pat. No. 5,135,732 and U.S. Pat. No. 4,246,253. However these materials exhibit problems, for example cycle fading (depletion in charge capacity over repeated charge/discharge cycles), which make them commercially unattractive. Attempts to address cycle fading have led to lithium metal phosphate and lithium metal fluorophosphates becoming favourable materials. Such materials were first reported in U.S. Pat. No. 6,203,946, U.S. Pat. No. 6,387,568, and by Goodenough et al. in “Phospho-olivines as Positive-Electrode Materials for Rechargeable Lithium Batteries”, Journal of Electrochemical Society, (1997) No. 144, pp 1188-1194.
Many workers have tried to provide economical and reproducible synthesis methods for phosphate-containing materials, and especially for high performance (optimised) phosphate-containing materials. A review of the prior art methods which describe the preparation of one particular lithium metal phosphate, namely, lithium iron phosphate (LiFePO4), is given by X. Zhang et al in “Fabrication and Electrochemical Characteristics of LiFePO4 Powders for Lithium-Ion Batteries”, KONA Powder and Particle Journal No. 28 (2010) pp 50-73. As this review demonstrates, a lot of effort has been expended since lithium iron phosphate was first identified in 1997, to find the most expedient method for producing a LiFePO4 material with the best all round performance; for example solid-state synthesis using mechanochemical activation to increase the activation of the starting materials, microwave heating to control the particle size of the active cathode material, and carbothermal reduction which enables Fe(III) e.g. in the form of Fe2O3 or FePO4 (i.e. cheap and readily available sources of iron) to be used as a precursor material. The carbothermal reduction process is a high-temperature reduction reaction (typically 550° C. to 850° C.) which commonly utilizes carbon black, graphite or pyrolyzed organic chemicals as the source of carbon reducing agent. Carbothermal reduction is a highly endothermic reaction; hence the reaction temperature must be sufficient to drive the reaction. In addition, since solid carbon is used as the reducing agent, all the precursors and reactants must be kept in good contact throughout the reaction, nevertheless as reported in the review mentioned above, carbothermal reduction is excellent for the reduction of Fe(III), the stabilization of Fe(II), the control of particle morphology, and the enhancement of electrical conductivity by coating LiFePO4 with residual carbon.
Particulate reducing agents other than carbon, specifically silicon oxide, titanium oxide and elemental metals such as Fe, Co, Ni, Mn, Cu, V, Ti, Cr, Nb, Mo, Mg, Ca, Zn, Sr, Pb, Cd, Sn, Ba, Be, Al and B are disclosed in EP1 343 720.
More recently US 2012/0061612 A1 discloses the synthesis of olivine-type lithium-containing phosphate compounds (LiMPO4) using a phosphorus-source such as at least one selected from phosphates, hypophosphites, phosphites and metaphosphates. The described reaction process involves a two stage firing regime, in which a first firing step is conducted at a temperature of higher than 400° C. and is for the removal of the volatile components. Preferably this first firing step is conducted in an atmosphere that contains 1 volume % or more of oxygen, and to support this all of the Specific Examples in this prior art are conducted under an air atmosphere. No Specific Examples using a hypophosphite are given however, and if hypophosphite were to be used in air or a 1 volume % oxygen atmosphere, one would discover that under these conditions the hypophosphite materials would oxidize to a phosphate material and that such a phosphate would have no effect in the process of the present invention.
The most interesting cathode materials are those which have large charge capacity, are capable of good cycling performance, highly stable, and of low toxicity and high purity. To be commercially successful, the cathode materials must also be easily and affordably produced. This long list of requirements is difficult to fulfil but, as detailed in the review mentioned above, the active materials most likely to succeed are those with small particle size and narrow size distribution, with an optimum degree of crystallinity, a high specific surface area and with uniform morphology.
Materials containing hypophosphite ions (H2PO2−), such as sodium hypophosphite (NaH2PO2) and ammonium hypophosphite (NH4H2PO2) are available very cheaply and widely used as the in-situ reducing agent for electroless plating baths; they reduce metal salts (e.g. nickel salts) to elemental metal (e.g. Ni) in the plating process. Other agents, for example materials containing hydrosulfite ions, such as sodium hydrosulfite (Na2S2O4) (also referred to as sodium hyposulfite and sodium dithionite), sodium sulfite (Na2SO3), formaldehyde, dimethyl amine borane (DMAB), hydrazine, aliphatic alcohols, trimethyl borane, and borohydride materials are also known to be powerful reducing agents.
The present invention aims to provide a fast, reliable and cost effective process for the preparation of metal-containing compounds and including but not limited to alkali metal-containing compounds. Advantageously, the process of the present invention aims to provide metal-containing compounds that meet the structural and alkali ion insertion properties needed for commercially viable cathode active materials. To this end, the present invention provides a solid state process for the preparation of a metal-containing compound comprising using a reaction mixture comprising i) one or more hypophosphite-containing materials and ii) one or more metal-containing precursor compounds.
In particular the present invention provides a solid state process for the preparation of a metal-containing compound comprising forming a reaction mixture comprising i) one or more metal-containing precursor compounds and optionally one or more non-metal-containing reactants and ii) one or more hypophosphite-containing materials; wherein one or more of the hypophosphite-containing materials is used as an agent to reduce one or more of the metal-containing precursor compounds; and further wherein the process is performed in the absence of an oxidising atmosphere.
The one or more metal-containing precursor compounds and the one or more metal-containing compounds preferably comprise one or more metals selected from alkali metals, transition metals, non-transition metals, alkaline earth metals and metalloids. In the context of this invention the term “metalloid” is an element with both metal and non-metal characteristics.
Ideally, the present invention provides a solid state process in which the metal-containing compound comprises one or more metals which have an average oxidation state which is lower than the average oxidation state of the one or more metals in the metal-containing precursor compounds. Further ideally, one or more of the metal-containing compounds and metal-containing precursor compounds comprise one or more transition metals. In this latter case, during the process of the present invention, one or more of the transition metals in the metal-containing precursor compounds is reduced by one or more of the hypophosphite-containing materials to yield a metal-containing compound with one or more transition metals with a lower average oxidation state than the average oxidation state of the one or more transition metals in the metal-containing precursor compounds.
In a highly preferred solid state process, the present invention produces compounds containing one or more alkali metals. Such compounds include alkali metal (metal)-containing compounds and may be produced for example by forming a reaction mixture comprising i) one or more metal-containing precursor compounds comprising alkali metals and optionally further metal-containing precursor compounds comprising one or more metals selected from transition metals and/or non transition metals and/or alkaline earth metals and/or metalloids, and further optionally one or more non-metal-containing reactants, and ii) one or more hypophosphite-containing materials; wherein one or more of the hypophosphite-containing materials are used as an agent to reduce one or more of the metal-containing precursor compounds; and further wherein the process is performed in the absence of an oxidising atmosphere.
The further metal-containing precursor compounds that comprise one or more metals selected from transition metals and/or non transition metals and/or alkaline earth metals and/or metalloids may also be used especially, but not exclusively, when the alkali metal-containing precursor compound does not already comprise a transition metal, and/or non-transition metal and/or alkaline earth metal and/or metalloid.
As described below, the alkali metal(metal)-containing compounds may alternatively be produced by forming a reaction mixture comprising one or more metal-containing precursor compounds and further optionally one or more non-metal-containing reactants, and ii) one or more alkali metal hypophosphite-containing materials; wherein one or more of the alkali metal hypophosphite-containing materials are used as an agent to reduce one or more of the metal-containing precursor compounds and to provide a source of alkali metal in the alkali metal (metal)-containing compounds; and further wherein the process is performed in the absence of an oxidising atmosphere.
A preferred solid state process of the present invention produces a metal-containing compound of the formula:AaMb(XcYd)eZf 
and comprises the step of forming a mixture comprising i) one or more metal-containing precursor compounds and optionally one or more non-metal-containing reactants, and ii) one or more hypophosphite-containing materials;
wherein:
A is an alkali metal selected from one or more of lithium, sodium and potassium;
M comprises one or more metals selected from transition metals, non-transition metals and metalloids;
(XcYd)e is at least one first anion; and
Z is at least one second anion
wherein a≥0; b>0; c>0; d≥0; e>0 and f≥0;
wherein a, b, c, d, e and f are chosen to maintain electroneutrality;
and wherein one or more of the hypophosphite-containing materials is used as an agent to reduce at least a portion of one or more of the metal-containing precursor compounds;
and further wherein the process is performed in the absence of an oxidising atmosphere.
Desirably, the solid state process of the present invention produces a metal-containing compound, for example of the formula AaMb(XcYd)eZf, in which M comprises one or more transition metals and/or non transition metals and/or alkaline earth metals and/or metalloids which have an average oxidation state which is lower than the average oxidation state of the one or more metals (transition metals and/or non transition metals and/or alkaline earth metals and/or metalloids) in the metal-containing precursor compounds.
The most preferred metal-containing compounds produced by the solid state process of the present invention are of the formula:AaMb(XcYd)eZf 
wherein:
A is an alkali metal selected from one or more of lithium, sodium and potassium;
M comprises one or more transition metals and optionally one or more further metals selected from non-transition metals and metalloids;
(XcYd)e is at least one first anion; and
Z is at least one second anion
wherein a≥0; b>0; c>0; d≥0; e>0 and f≥0;
wherein a, b, c, d, e and f are chosen to maintain electroneutrality;
and wherein the process comprises forming a mixture comprising i) one or more metal-containing precursor compounds and optionally one or more non-metal-containing reactants, and ii) one or more hypophosphite-containing materials; wherein one or more of the hypophosphite-containing materials is used as an agent to reduce one or more of the metal-containing precursor compounds;
and further wherein the process is performed in the absence of an oxidising atmosphere.
The addition of one or more hypophosphite-containing materials as a reducing agent (e.g. they reduce the oxidation state of the metal, particularly a transition metal, of the metal-containing precursor compounds), is crucial to the success of the invention and depending upon the particular one or more metal-containing precursor compounds, and the desired final product, the hypophosphite-containing materials may additionally behave as a source of phosphorus in the final product and/or as a source of the alkali metal. For example, when the target metal-containing compound contains sodium and phosphorus, both of these elements may potentially be obtained from sodium hypophosphite when the latter is used as the reducing agent. The fact that the one or more hypophosphite-containing materials potentially acts as both the reducing agent and/or as a source of phosphorus and/or as a source of alkali metal in the metal-containing compound is seen as one of the many advantages of the present invention.
In a particularly preferred process, alkali metal (metal)-containing compounds are prepared. Such compounds may have the formula AaMb(XcYd)eZf where A is one or more alkali metals, M comprises one or more transition metals and/or one or more non transition metals and/or one or more alkaline earth metals and/or one or more metalloids, and X, Y, and Z are as defined below. In such alkali metal (metal)-containing compounds, a>0, b>0, d≥0, e>0 and f≥0.
In the reaction products produced by the solid state process of the present invention:
A preferably comprises one or more alkali metals selected from sodium, lithium and potassium;
M comprises one or more metals selected from transition metals such as titanium, vanadium, niobium, tantalum, hafnium, chromium, molybdenum, tungsten, manganese, iron, osmium, cobalt, nickel, palladium, platinum, copper, silver, gold, zinc, cadmium, aluminum, scandium, yttrium, zirconium, technetium, rhenium, ruthenium, rhodium, iridium, mercury, gallium, indium, tin, lead, bismuth and selenium, non transition metals and alkaline earth metals such as magnesium, calcium, beryllium, strontium and barium, and metalloids such as boron, silicon, germanium, arsenic, antimony and tellurium;
X comprises one or more elements selected from titanium, vanadium, chromium, arsenic, molybdenum, tungsten, niobium, manganese, aluminum, selenium, boron, oxygen, carbon, silicon, phosphorus, nitrogen, sulfur, fluorine, chlorine, bromine and iodine.
Y comprises one or more halides, sulfur-containing groups, oxygen-containing groups and mixtures thereof;
Z is selected from one or more halides, hydroxide-containing groups and mixtures thereof.
Any hypophosphite-containing materials may be used in the present invention, however, preferably they are any one or a mixture of materials selected from lithium hypophosphite (LiH2PO2), sodium hypophosphite (NaH2PO2), ammonium hypophosphite (NH4H2PO2) and hypophosphorus acid (H3PO2). The most preferred materials comprise sodium hypophosphite (NaH2PO2) and/or ammonium hypophosphite (NH4H2PO2).
Desirable compounds of the formula AaMb(XcOd)eZf include, but are not limited to, those in which A is lithium and/or sodium, and in which the first anion (XcYd)e comprises one or more groups, preferably selected from phosphate, condensed polyphosphate, sulfate, oxide, thiosulfate, sulfite, chlorate, bromate, oxyhalide, halide, silicate, arsenate, selenate, molybdate, vanadate groups and any oxyanion groups.
Compounds where X comprises phosphorus, for example in which (XcYd)e is a PO4 and/or P2O7 moiety are especially preferred. Similarly, compounds in which X comprises sulfur are equally advantageous, such as those containing SO4 moieties.
Other favourable materials include:
LiFePO4,
LiFePO4/Fe2P,
LiMnPO4,
LiCoPO4,
LiNiPO4,
NaFePO4,
NaMnPO4,
NaCoPO4,
NaNiPO4,
LiMn0.5Fe0.2Mg0.3PO4,
Li3V2(PO4)3,
Na4Fe3(PO4)2P2O7,
Na3V2(PO4)3,
LiMn0.5Fe0.5PO4,
Na7V4(P2O7)4PO4,
Na7V3(P2O7)4,
Na2Fe(SO4)2,
NaVPO4F,
LiVPO4F,
Na3V(PO4)2,
Li3V(PO4)2,
NaVOPO4,
LiVOPO4,
LiV2O5,
NaV2O5,
NaVO2,
VPO4,
MoP2O7,
MoOPO4,
Fe3(PO4)2,
Na8−2xFe4+x(P2O7)4,
Na8−2xMn4+x(P2O7)4,
Na2MnP2O7,
Na2FeP2O7,
Na2CoP2O7,
Na4Mn3(PO4)2P2O7,
Na4Co3(PO4)2P2O7,
Na4Ni3(PO4)2P2O7,
NaFeSO4F,
LiFeSO4F,
NaMnSO4F,
LiMnSO4F,
Na2FePO4F,
Na2MnPO4F,
Na2CoPO4F,
Na2NiPO4F,
Na2Fe2(SO4)3,
Li2Fe2(SO4)3, and
Li2Fe(SO4)2.
Advantageously, the present invention provides a solid state process for the preparation of metal phosphate-containing materials (MPO4) comprising using one or more hypophosphite-containing materials, (for example sodium hypophosphite (NaH2PO2) and/or ammonium hypophosphite (NH4H2PO2)) as an agent to reduce at one or more metal-containing precursor compounds and optionally also as a source of phosphorus; wherein the process is performed in the absence of an oxidising atmosphere.
In particular, the present invention provides a solid state process for the preparation of a compound comprising an alkali metal (metal) phosphate of the general formula: AMPO4, where A comprises one or more alkali metals; M comprises a metal selected from one or more of manganese, iron, cobalt, copper, zinc, nickel, magnesium and calcium, the process comprising forming a mixture of i) one or more metal-containing precursor compounds and optionally one or more non-metal-containing reactants and ii) one or more hypophosphite-containing materials; wherein one or more of the hypophosphite-containing materials is used as an agent for reducing at least a portion of one or more of the metal-containing precursor compounds and optionally also as a source of phosphorus and/or source of alkali metal; further wherein the process is performed in the absence of an oxidising atmosphere.
Preferred AMPO4-containing compounds include NaMPO4 and LiMPO4; LiFePO4-containing compounds are particularly preferred. These compounds may be produced in the solid state process of the present invention by forming a mixture comprising i) one or more metal-containing precursor compounds and optionally one or more non-metal-containing reactants; and ii) one or more hypophosphite-containing materials; wherein one or more of the hypophosphite-containing materials are used as an agent to reduce one or more of the metal-containing precursor compounds, wherein component i) of the mixture may comprise one or more compounds selected from LiH2PO4, Li2HPO4, LiOH, LiOH.H2O, Fe3O4, Fe2O3, Li2CO3, FePO4.x H2O, FePO4, Fe3(PO4)2, FeSO4.x H2O, Fe(NO3)3, Fe(CH3CO2)2, C6H8O7.x Fe3+.y NH3 (ammonium iron (III) citrate), C6H5FeO7 (iron (III) citrate) and Fe(C5H7O2)3 (iron (Ill) 2,4-petanedionate) and further wherein the process is performed in the absence of an oxidising atmosphere.
NaMPO4 may be produced via a similar reaction process wherein the metal-containing precursor compounds may comprise one or more compounds selected from NaH2PO4, Na2HPO4, NaOH, Fe3O4, Fe2O3, Na2CO3, FePO4.x H2O, FePO4, Fe3(PO4)2, FeSO4.x H2O, Fe(NO3)3, Fe(CH3CO2)2, C6H8O7.x Fe3+.y NH3 (ammonium iron (III) citrate), C6H5FeO7 (iron (III) citrate) and Fe(C5H7O2)3 (iron (III) 2,4-petanedionate)
The reaction mixture in either case may also include any other suitable reagents and/or sources of phosphorus such as H3PO4, (NH4)2HPO4, (NH4)H2PO4.
A convenient way to perform the solid-state process of the present invention is by:
1. forming a mixture comprising i) one or more metal-containing precursor compounds and optionally one or more non-metal-containing reactants, and ii) one or more hypophosphite-containing materials;
2. heating the mixture under a non-oxidizing atmosphere; and
3. recovering the resultant product, preferably a metal-containing compound of the formula:AaMb(XcYd)eZf 
wherein A is one or more alkali metals selected from lithium, sodium and potassium, M comprises one or more metals selected from transition metals, non transition metals, alkaline earth metals and metalloids, (XcYd)e is at least one first anion and z is at least one second anion, and wherein a≥0, b>0, c>0, d≥0, e>0 and f≥0.
Ideally the starting materials are intimately admixed in particulate form. This can be achieved using various methods, for example by finely grinding the materials separately using a pestle and mortar or a ball mill, and then mixing them together, or the materials can be admixed whilst they are being finely ground. The grinding and admixing is of sufficient duration to produce a uniformly intermixed finely ground powder. A solvent such as acetone or another material which is easily removed, for example a low boiling liquid, can be used to assist the grinding/admixing process and this is preferably removed prior to the heating step. Other known techniques such as high energy ball milling and microwave activation may also be used to help prepare the starting materials, for example to increase their reactivity.
A key feature of the present invention is that it is a “solid-state” reaction i.e. a reaction in which all of the reactants are in solid form and are substantially free of any reaction medium such as a solvent. Where a solvent or other low boiling liquid is used to assist the mixing of the reactants, as described above, it is substantially removed prior to the heating step. Based on prior art knowledge, it is known that hypophosphite-containing materials are useful reducing agents when employed in alkaline solution reactions, particularly as the soluble chemical reducing agent in electroless nickel and copper baths. However, the use of hypophosphite-containing materials is not yet known for the reduction of metals to intermediate oxidation states (i.e. not to the metallic state), and it is as yet unknown that they are effective reducing agents for metals, particularly transition metals, other than copper and nickel. Moreover, it is highly surprising that hypophosphite-containing materials are effective reducing agents in solid state reactions, and that in such reactions they can also provide some or all of the phosphate and/or the alkali metal component of phosphate- and/or alkali metal-containing products.
The reaction between the starting materials generally occurs during the heating step of the process. This typically involves heating the reaction mixture either at a single temperature, or over a range of temperatures, for example up to at least 150° C., preferably up to at least 200° C. A single or a range of reaction temperatures of from 150° C. to 1200° C. is preferred with from 150° C. to 800° C. being particularly preferred.
Conveniently the reaction is performed under atmospheric pressure and it must be conducted under a non-oxidising atmosphere, for example nitrogen, argon or another inert gas, or under vacuum, and depending on the target material and the precursors used, the reaction may also be performed in a sealed reaction vessel. All of these reaction such conditions are within the scope of the present invention and are all to be included in the definition of “non-oxidizing atmosphere” and/or “absence of an oxidizing atmosphere”.
Advantageously, the reaction temperature is maintained for between 0.5 and 12 hours, although the exact time will depend on the reactivity of the starting materials. Between 0.5 and 8 hours has been found to be sufficient for many reactions utilising the process of the present invention.
As discussed above, in the process of the present invention the one or more hypophosphite-containing materials are agents for the reduction of the average oxidation state of the metal (transition metal and/or non transition metal and/or alkaline earth metal and/or metalloid) in the one or more metal-containing precursor compounds, and may additionally provide a source of phosphorus and/or a source of alkali metal.
In a preferred reaction scheme, lithium iron phosphate is prepared according to the process of the present invention from:0.125 Li2CO3+0.25 NH4H2PO2+0.75 LiH2PO4+0.5 Fe2O3→LiFePO4+0.125 CO2+0.25 NH3+1.125H2O
The metal-containing materials of formula AaMb(XcYd)eZf prepared by the process of the present invention are suitable for use in many different applications, for example as the active material in electrodes, particularly cathodes used in energy storage devices, rechargeable batteries, electrochemical devices and electrochromic devices. This is especially the case for alkali metal (metal)-containing materials. Advantageously, the electrodes made using the materials produced by the present invention are used in conjunction with a counter electrode and one or more electrolyte materials. The electrolyte materials may be any conventional or known materials and may comprise either aqueous electrolyte(s) or non-aqueous electrolyte(s).
An inherent problem with a number of metal-containing compounds, especially alkali metal-containing compounds, is their low electrical conductivity. To address this problem it is known to add conductive materials such as carbon-containing materials for example, graphite, carbon black, sucrose and acetylene black either to the starting materials, such as during grinding, or as a coating to the final metal-containing products. Other known conductive materials include metal powders and other highly conductive inorganic materials.
It is therefore desirable, when making inherently non-conductive materials such as alkali metal-containing compounds using the solid state process of the present invention, to add/intimately disperse one or more conductive materials such as carbon to the reaction mixture and/or to one or more of the starting materials and/or to the final product.
It is believed that the AaMb(XcYd)eZf compounds produced by the process of the present invention may, where the starting materials so favour, be in the form of a composite material that includes a conductive compound which is preferably produced in situ during the reaction between the metal-containing precursor compounds and one or more hypophosphite-containing materials. The conductive compound formed in situ is preferably a phosphorus-containing compound, and a suitable conductive material may comprise, at least in part, a transition metal phosphide- and/or a non-transition metal phosphide- and/or a metalloid phosphide-containing material such as, in the case where the metal component M comprises iron, an iron phosphide-containing material, for example Fe2P. This latter iron phosphide material in particular, is known to be highly conductive.
Thus in a second aspect, the present invention provides a composition comprising a metal-containing compound e.g. of the formula AaMb(XcYd)eZf as defined above, and one or more conductive materials, wherein at least a portion of the one or more conductive materials is formed in situ during the solid state process described above, in particular when one or more of the hypophosphite-containing materials is used as an agent to reduce one or more of the metal-containing precursor compounds. Desirably the invention provides a composition comprising LiFePO4 and at least one conductive material comprising one or more phosphide-containing compounds. Suitable phosphide-containing compounds may include, but are not limited to binary phosphides.
Further, in a third aspect, the present invention provides a process for preparing a composition comprising a metal-containing compound, e.g. of the formula AaMb(XcYd)eZf defined as above, comprising forming a reaction mixture comprising i) one or more metal-containing precursor compounds and optionally one or more non-metal-containing reactants, and ii) one or more hypophosphite-containing materials; wherein one or more of the hypophosphite-containing materials is used as an agent to reduce one or more of the metal-containing precursor compounds; and further wherein the process is conducted in the absence of an oxidizing atmosphere.
In a fourth aspect, the present invention provides an electrode which utilises active materials of formula AaMb(XcYd)eZf, prepared in accordance with the present invention as described above, especially an electrode which utilises a composition comprising such active materials in combination with a phosphorus-containing conductive material, and particularly a phosphorus-containing conductive material which has been made, at least in part, during the reaction process described above involving one or more hypophosphite-containing materials.
In a another aspect, the present invention provides a solid state method for the preparation of metal-containing compounds comprising forming a mixture of i) one or more metal-containing precursor compounds and ii) one or more materials selected from hydrosulfite-containing materials, sulfite-containing materials, formaldehyde, dimethyl amine borane (DMAB), hydrazine and borohydride-containing materials.
The hydrosulfite-containing materials may comprise, for example, sodium hydrosulfite (Na2S2O4) (also referred to as sodium hyposulfite and sodium dithionite) or lithium hydrosulfite (Li2S2O4), and the sulfite-containing materials may comprise, for example, sodium sulfite (Na2SO3) or lithium sulfite (Li2SO3). The latter sulfite-containing compounds are used as reducing agents, however, they advantageously may also provide a source of the sulfur and/or alkali metal component of the reaction product, for example where the reaction product is an alkali metal and/or sulfate-containing material. Thus the present invention provides a solid state process for the preparation of sulfate-containing materials comprising using one or more hydrosulfite-containing materials and/or sulfite-containing materials as a reducing agent and additionally as a source of sulfur and/or source of alkali metal. The reaction is performed in the absence of an oxidising atmosphere.
In still further aspects, the present invention provides an energy storage device comprising an electrode as described above, for use as one or more of the following: a sodium ion and/or lithium ion and/or potassium ion cell; a sodium metal and/or lithium metal and/or potassium metal ion cell; a non-aqueous electrolyte sodium ion and/or lithium ion and/or potassium ion cell; and an aqueous electrolyte sodium ion and/or lithium ion and/or potassium ion cell. Specifically, the energy storage device may be a battery.